Unidirectional grained ferrous alloy containing aluminum



United States Patent Olfice 3,528,805 Patented Sept. 15, 1970 3,528,805 UNIDIRECTIONAL GRAINED FERROUS ALLOY CONTAINING ALUMINUM John Harrison, Sheffield, England, assignor to Swift Levick & Sons Limited No Drawing. Continuation-in-part of application Ser. No. 339,320, Jan. 22, 1964. This application Apr. 17, 1967, Ser. No. 631,134

Int. Cl. C33c 39/02 US. Cl. 75-124 5 Claims ABSTRACT OF THE DISCLOSURE An anisotropic columnar crystal magnet consisting eshas magnetic properties of at least: Bram, 9800 gauss; H,, 1200 oersteds; and (BH) 6.0 gauss-oersteds.

This is a continuation-in-part of my prior application, Ser. No. 339,320, filed Jan. 22, 1964, now Pat. No. 3,314,828.

This invention relates to permanent magnets of the Fe-Al-Ni-Co-Cu type and is essentially concerned with the high-energy forms of this type based on a columnar crystal structure. The general composition by weight, of alloys believed to be capable of yielding a beneficial columnar structure, can be taken as regards these main constituents named, as embraced by:

Percent Al 5-1 1 Ni 7-25 Co -40 Cu 0-10 Fe Balance The balance however usually comprises, additionally to Fe, minor amounts of other constituents present as unavoidable impurities, not having any significant effect on the magnetic properties. More importantly, it may also comprise constituents deliberately introduced in significant (but not necessarily substantial) amount for the improvement of one or more of the ultimate magnetic properties, or the adjustment of the magnetic characteristics as delineated by the demagnetisation curve.

Thus Ti has long been known to lead to an increase in coercivity. Nb and Ta (usually available in association, and therefore generally considered in terms of the amount of Nb actually present) may replace part of the Ti present with that object, the Nb and Ta conferring beneficial heat treatment characteristics on the high coercivity alloys. Nb and/or Ta in the absence of Ti will not impart coercivity of the high order obtainable with Ti. However, Nb (and/ or Ta) are conducive to the formation of a colummar crystal structure, with which the greatest benefits of anisotropy are associated when the crystallisation is in a preferred direction of magnetisation.

Yet again, the balance may include constituents similarly present by deliberation for the improvement of the mechanical properties of the magnet alloys, notably S for amelioration of the brittleness that is a characteristic of these alloys, to an extreme degree with some compositions. However, S differs from some other minor constituents the presence of which below a restricted individual or collective maximum can be accepted as impurity," if it is not brought about by deliberate addition, e.g., P, Zr, Cr, Mn, C, and Si, which can be designated as benign because of their lack of effect on the magnetic properties, even though some adjustment of the heat treatment may be necessary to allow for, or to take advantage of, their inclusion in the alloy, whereas such minor constituents are taken into solution, S is not, and its presence above the trace level is not without effect on the magnetic properties. As the S content increases above that level, so do the magnetic properties ordinarily decrease, this being believed to be due to the formation of non-magnetic sulphides in the alloy and a consequent dilution effect.

It is, however, by no means the case that all combinations of such different constituents, minor or otherwise, each tending to some improvement in the final magnets, can contribute as a useful total to the balance, consisting predominantly of Fe, of a particular composition of the main constituents: incompatibility of one sort or another is known to arise. In particular, Ti, in the ordinary way is definitely not conducive to the formation of a columnar crystal structure, when using an exothermic mould with a chill plate, and it has been found that with Ti present to any appreciable extent, elaborate and expensive methods of casting have necessarily had to be employed to produce a columnar crystal structure in such alloys.

Because of these considerations, whilst the highest energy product (BH) of commercially manufactured anisotropic materials has been brought into the range 7 to 8 10 gauss-oersteds by the columnar crystal technique, the coercivity H has been found to be in the range 700 to 800 oersteds. That coercivity is much below the 1000 to 1200 oersteds or more obtainable, at the expense of much lower energy (5X10 gauss-oersteds) and remanence, by an appropriate amount of Ti, Nb, or Ta, in magnets to which the columnar crystal technique is inapplicable.

High coercivity and high energy have up to the moment only been obtainable simultaneously by the application of a special technique on special materials, not applicable to the production of commercial magnets. Thus (see Phillips Research Reports 11, 1956, pages 489 to 490), rods of pure alloy.

Percent Ni 15 A1 a- 7 Co 34 Cu 4 Fe 35 Ti 5 prepared by melting pure metals in pure argon were used for the preparation by a crystal-pulling technique of single crystals having an axis nearly parellel to the direction of pulling, with the following properties:

(BH) -l 1 X 10 gauss-oersteds B, 11,800 gauss H,, -l315 oersteds.

In the report just referred to reference is made to the decrease in (BH) resulting from the use of Ti to attain high H In Cobalt, December, 1961, Wright and Thomas conclude that it is the combination of Ti and Al together, in various compositions falling within what is usual for magnet alloys of the type now in question, that leads to fine equi-axed crystals in magnets subjected to the chilling technique as employed for the production of columnar crystal magnets.

Contrary to the evidence of existing practice and belief, the present invention is based on the discovery that the apparent incompatibility of Ti as regards simultaneous attainment of high energy and high coercivity can be overcome, and overcome without significant departure from the ordinary commercial method of producing permanent magnets, using an exothermic mould with a chill plate.

The discovery is that S, (which is ordinarily considered as improving only the mechanical properties), present in significant but limited amount in conjunction with Ti, or with both Ti and Nb (Cb), enables energy and coercivity, to be simultaneously achieved to values not previously attainable in commercial magnets.

According to the present invention, an anisotropic columnar crystal magnet has the composition by weight:

Percent Al 6.5-8 Ni 13-17.5 Co 27-40 Cu 2-5 Ti 3.5-6 Nb (Cb) -3 Si Up to 1 S 0.125-1 Fe and impurities balance.

Preferably, the composition is:

Percent Al 6.5-7.5 Ni 14-15 Co 27-35 Cu 3-5 Ti 4-5.5 Nb (Cb) 0-2 Si Up to 0.5 S 0.15-0.25

Fe and impurities balance It is an important feature of the invention that the magnetic properties obtained with alloys falling within the broad range stated are not less than:

and, as shown by specific examples given below, such combination of high properties are only attainable when the alloys are cast by commercial production methods, if S is present Within the range stated.

As shown by specific examples given below, alloys falling within the preferred composition are found to have magnetic properties of the following order:

B, 10000-1l500 gauss H mainlyl200-1650 oersteds (BH) 6.0-11.5 X gauss-oersteds when cast by the usual chilling technique and subjected to magnetic cooling and tempering as ordinarily accorded to the type of alloys used for columnar crystal magnets.

It is believed that the S forms a barrier of immiscible liquid sulphides between certain crystal and nuclei and the melt in the case of magnet alloys containing Ti, to overcome the tendency of the nuclei to promote rapid random crystallisation throughout the cooling liquid, and thus to permit crystallisation to proceed by columnar growth.

The S may be conveniently introduced as ferrous sulphide. The introduction is best effected with the main ingredients of the intial charge to be melted, rather thanas is done with the Alat or towards the end of the melting operation.

The examples of which particulars are given below show the magnetic properties obtained by the addition of S in columnar crystal magnets with various compositions within the ranges specified previously. In the case of Examples 1 and 2, and 3 and 4, Examples 2 and 4 are the same alloys as Examples 1 and 3 respectively except in that the amount of S present is insufficient to overcome the long known tendency of Ti to cause the formation of an equi-axed crystal structure. The alloys of Examples 1 and 2, and 3 and 4 were not only cast in the same manner, i.e., in an exothermic mould provided with a chill plate, but were also given identical heat treatments. For Examples 1 and 2, the heat treatment was:

Solution temperature: 1250 C.

Fast cool (salt bath quench): to 810 C.

Maintain at 810 C. (:5" C.) for 10 mins. in a magnetic field of 3000 oersteds Air cool to room temperature Temper: 590 C. for 48 hours, followed by 560 C. for

48 hours.

For Examples 3 and 4, the heat treatment was:

Solution temperature: 1250 C.

Blast cool for 1 /2 to 2 mins.

Slow cool for 10 mins. in a magnetic field of at least 3000 oersteds Temper: 590 C. for 48 hours, followed by 560 C. for

48 hours.

The remaining examples were all given the heat treatment:

Solution temperature: 1250 C.

Fast cool (air blast): to 1000" C. in 2 mins.

Magnetic cooling: to 600 C. in 10 mins.

Temper: 590 C. for 48 hours, followed by 560 C. for

48 hours.

Example 5 is a further alloy with S present in an amount insufiicient to overcome the effect of Ti, there being no example of that alloy with suflicient amount of S to allow a columnar crystal formation, and no magnetic properties are given for Examples 6 to 12 when these alloys have an insufficient amount of S and thus an equi-axed crystal structure, but the drop in magnetic propperties is as marked with these examples (particularly (BH) as is the case with Examples 2 and 4.

EXAMPLES Compositions (by analysis) by weight of constituents stated, the balance being Fe and impurities.

Al Ni C0 C11 Ti Nb (Cb) Si S (The slight differences between Examples 1 and 2, and 3 and 4-except for S-are due to analytical error.)

EE designates an equiaxed crystal structure. CC designates a columnar crystal structure.

The manner in which the rate of cooling during the application of the magnetic field is controlled depends upon the size of the magnet castings. If desirable, the cooling immediately following the initial soaking at solution temperature may be appropriately accelerated by an air blast, say into the range 970 C. to 1000 C. How ever, complete cooling (or at least to black heat), preferably in a magnetic field, may be used, followed by re-heating in the magnetic field to say 825 C. and holding the magnet at that temperature while maintaining the field.

The field strength in the usual range of 1000 to 2000 oersteds is effective in carrying out the invention, but a field strength of at least 3000 oersteds has been found advantageous, and the use of still higher strengths of 7000 oersteds and upwards is not precluded.

As already indicated, Si up to 1.0% by weight may be present, but it appears that the higher the Si the lower should be the Al in its stipulated range, and also that higher field strength in cooling is then likely to be beneficial.

What I claim is:

1. An anisotropic unidirectionally grained columnar crystal magnet consisting essentially of:

Percent Al 6.5-8 Ni 13-17.5 Co 27-40 Cu 2-5 Ti 3.5-6 Nb (Cb) -3 Si Up to 1 S 0.125-1 Fe and impurities balance.

2. An anisotropic unidirectionally grained columnar crystal magnet consisting essentially of:

Percent A1 6.5-7.5 Ni 14-15 Co 27-35 Cu 3-5 Ti 4-5.5 Nb (Cb) 0-2 S1 Up to 0.5 S 0.15-0 25 Fe and impurities balance.

3. A anisotropic unidirectionally grained columnar crystal magnet as claimed in claim 1 having magnetic properties of at least: B 9800 gauss, H 1200 oersteds; and (BH),,,,,,, 6.0 10 gauss-oersteds.

4. An anisotropic unidirectionally grained columnar crystal magnet as claimed in claim 3 having magnetic properties in the approximate ranges: B 10000 to 11500 gauss; H 1200 to 1650 oersteds; (BHL 6.0 to 11.5 X 10 gauss-oersteds.

5. An anisotropic unidirectionally grained columnar crystal magnet consisting essentially of:

About (percent) Al 6.9 Ni 14.4 C0 33.5 Cu 3.85 Ti 5.45 Si 0.26 S 0.214

Fe and impurities balance.

having magnetic purities: Brem, about 11100 gauss; H about 1620 oersteds; and (BH) about 11.5)(10 gauss-oersteds.

References Cited UNITED STATES PATENTS 2,797,161 6/1957 Ireland 124 3,085,036 4/1963 Steinort 75-124 X 3,175,901 3/1963 Jesmont 75-124 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 1483, 31.57 

